The present invention relates to a printing machine and method and, more particularly, to a method and system for printing in accordance with offset printing techniques employing an intermediate medium referred to as a blanket to transfer an image from a printing plate to a sheet or web.
In printing machines of the type operating on an offset principal, ink is applied from an inking roller to a printing plate or other master mounted on a rotatable plate cylinder. A blanket mounted on a blanket cylinder adjacent the plate cylinder receives a film of ink in the form of an image from the plate cylinder as the plate or master is rotated in contact with the blanket. The image is then transferred to a sheet or web of material which passes between the blanket cylinder and an impression cylinder or platen.
The surfaces of the inking roller and the blanket employed in offset printing machines are typically composed of elastically deformable materials exhibiting good ink receptivity characteristics. For example, rubber and other suitable materials such as polyurethane and polyvinyl chloride have been employed for blankets and other ink receiving surfaces as is indicated in U.S. Pat. No. 3,765,329.
While acceptable results have been achieved using inking rollers and blankets having rubber or polymeric surfaces, numerous problems have also been encountered. For example, previously used materials provide relatively poor ink release and typically require complex or time consuming cleaning procedures to remove ink retained subsequent to the transfer process. A blanket having a rubber or other similar surface may, for example, retain as much as 50% of the ink film applied thereto when moved into contact with the sheet or web of material to which the image is being applied. Poor ink release not only detracts from the quality of the printing but also requires directing additional attention to the removal of the retained ink from the surface of the blanket.
Moreover, it is desirable that the blanket retain its elasticity so that the blanket does not become permanently impressed or engraved by the plate. Typical materials such as rubber now employed in the construction of blankets tend to swell due to the oils or other substances in the inks. The swelling of the blanket detracts from the printing properties of the blanket by, for example, increasing the printing pressure in one area and thereby causing the blanket to print heavier in that area than in other areas. Additionally, a swelling of the blanket reduces its strength and resiliency and the additional pressure between the blanket and both the plate and impression cylinders may cause the rubber surface to be engraved or otherwise damaged during the printing operation.
One attempt to obtain better ink release from a blanket in an offset printing machine has involved the use of polytetrafluoroethylene as the image receiving surface of the blanket. The use of polytetrafluoroethylene (e.g. Teflon .RTM.) is disclosed in U.S. Pat. No. 3,164,087 as a means for reducing the residual image remaining on the surface of the blanket after the printing (i.e. a means for improving ink release). While the use of polytetrafluoroethylene may provide better ink release, this material deforms plastically, i.e. has no elastic memory, and therefore becomes engraved by the plate. In addition, ink applied to a polytetrafluoroethylene blanket tends to form globules instead of continuous film as is desirable for achieving maximum quality in printing. Polytetrafluoroethylene blanket and inking roller surfaces are also objectionable because the ink receptivity of the polytetrafluoroethylene is inferior to conventional materials such as rubber.
It is accordingly an object of the present invention to obviate the foregoing problems associated with the use of rubber, polyurethane, polytetrafluoroethylene, and the like for inking roller and blanket surfaces.
It is another object of the present invention to provide a novel method and printing apparatus wherein ink receiving surfaces are not subject to swelling, are highly receptive to conventional inks and exhibit desirable ink release characteristics.
It is a more specific object of the present invention to provide a novel offset printing method and blanket having an image receiving surface of a fluoroelastomer which does not swell or become tacky with prolonged use, has an elastic memory so that it does not readily become engraved, and yet exhibits excellent ink receptivity and release properties.
It is yet a further object of the present invention to provide a novel method and inking roller having the properties of the blanket of the foregoing object.
These and other objects and advantages of the present invention will become apparent to one skilled in the art to which the invention pertains from the following detailed description when read in conjunction with the appended drawings.